1. Field of the Invention
The present invention relates to a spectrometer which is used to measure a spectrum of a light beam supplied from a light source, in particular, a spectrometer that can obtain, from a spectrum including specified wavelength components, fine information (information of a spectral component in a relatively narrow range in the vicinity of a peak of the spectrum) at high accuracy and coarse information (information of a spectral component in a relatively wide range around the peak, in which information of a foot part of the spectrum is important).
2. Description of a Related Art
A spectrometer disclosed in Japanese patent application publication JP-A-11-132848 is known as an optical instrument which is used to measure a spectrum of a light beam supplied from a light source. FIG. 10 shows constitution of the spectrometer.
The spectrometer as shown in FIG. 10 has slit board 100, collimator lens 101, beam splitter 102, diffraction grating 103, mirror 104, magnifier lens 105 and line sensor 106.
In the spectrometer, a light beam supplied from a light source passes through slit board 100 and is changed into a parallel light beam by collimator lens 101. In beam splitter 102, the parallel light beam is divided into a reflected light beam which travels toward collimator lens 101 and a transmitted light beam which travels toward diffraction grating 103. The transmitted light beam is incident into diffraction grating 103, and a part of the transmitted light beam is diffracted toward beam splitter 102 as a first diffraction light beam (a single pass beam).
A part of the single pass beam is reflected by beam splitter 102 toward a direction shifted by a slight angle from the optical axis of collimator lens 101. On the other hand, the rest of the single pass beam passes through beam splitter 102.
The single pass beam reflected by beam splitter 102 is incident into diffraction grating 103 again, and a part of the incident light beam is diffracted as a second diffraction light beam (a double pass beam) from diffraction grating 103 to beam splitter 102. A number of times of diffraction of the double pass beam is one more than that of the single pass beam, so that a dispersion value (a difference in wavelengths which corresponds to an interval of adjacent two channels of line sensor 106) of the double pass beam is small. Therefore, the double pass beam realizes high resolving power.
And then, a part of the double pass beam is reflected by beam splitter 102 toward a direction shifted by a slight angle from the optical axis of collimator lens 101. On the other hand, the rest of the double pass beam passes through beam splitter 102.
By the way, when an excimer laser source is used as a light source, according to performance of the excimer laser source and so on, a useless component may appear in a foot part of the spectrum which is distant from a peak of the spectrum, and a blur may arise by chromatic aberration of the optical system. Therefore, in measurement of a spectrum of a light source, it is necessary to carry out not only measurement in the vicinity of a peak of the spectrum with higher resolving power (fine measurement) but also measurement in a foot part of the spectrum with lower resolving power (coarse measurement).
In the conventional spectrometer as shown in FIG. 10, line sensor 106 can detect either a single pass beam or a double pass beam in general. FIG. 10 shows the case where line sensor 106 detects a single pass beam.
Therefore, two spectrometers which are different in a value of resolving power are prepared. At first, measurement of a spectrum of an excimer laser beam is carried out by using a spectrometer for the single pass beam, which has lower resolving power, so as to obtain coarse information of the spectrum. Next, the spectrometer for the single pass beam is replaced with a spectrometer for the double pass beam, which has higher resolving power, and measurement of the spectrum is carried out so as to obtain fine information of the spectrum. Further, a compound spectrum of the excimer laser beam is obtained by compounding the coarse information and the fine information together under a suitable processing.
As another way, two diffraction gratings which are different in the number of groove lines are prepared. At first, measurement of a spectrum of an excimer laser beam is carried out by installing a diffraction grating for the single pass beam to the body of the apparatus so as to obtain coarse information of the spectrum. Next, the diffraction grating for the single pass beam is replaced with a diffraction grating for the double pass beam, which is different from the diffraction grating for the single pass beam in the number of groove lines, and measurement of the spectrum is carried out so as to obtain fine information of the spectrum. Further, a compound spectrum of the excimer laser beam is obtained by compounding the coarse information and the fine information together under a suitable processing.
In the spectrometer disclosed in JP-A-11-132848, however, the following problems cause when the coarse information and the fine information in the spectrum of the excimer laser beam is obtained.
(a) The cost becomes higher because two spectrometers or two diffraction gratings need to be prepared for the single pass beam and the double pass beam.
(b) The process of measuring a spectrum becomes more complicated because one spectrometer or diffraction grating for the single pass beam needs to be replaced with the other for the double pass beam.
Therefore, it is proposed to enlarge a size of the line sensor in the longitudinal direction so as to focus the single pass beam and the double pass beam on channels of the line sensor (different channels are used for the single pass beam and for the double pass beam).
However, according to the above-mentioned proposal, either the single pass beam or the double pass beam is focused on a channel in the edge portion of the line sensor. As a result, the inaccurate detection result due to image shift may be obtained.
Further, it is also proposed to rotate a diffraction grating adequately so as to focus the single pass beam or the double pass beam on channels of a line sensor. That is, the single pass beam is focused on channels of the line sensor by rotating diffraction grating 103 from the Littrow arrangement, in which an incident angle is equal to an output angle, by a predetermined slight angle. Next, the double pass beam is focused on channels of the line sensor by rotating diffraction grating 103 from the Littrow arrangement by a predetermined slight angle (which is different from that in detecting a single pass beam).
However, according to the above-mentioned proposal, there is a problem that it is practically difficult to realize high resolving power without enlarging a size of the apparatus.
The resolving power of a spectrometer according to the above-mentioned proposal (which will be explained with referring to FIG. 10 in the following) becomes higher as a dispersion value in the line sensor becomes smaller. The dispersion value is defined by the following expression:
disp=sw/(fxc2x7angDisp) xe2x80x83xe2x80x83(1) 
Where each symbol represents the following value.
disp: a dispersion value
sw (=swd/mag): a one-to-one conversion size of the line sensor
swd: a size of the line sensor
mag: a magnification rate of a magnifier lens
f: a focal length of a collimator lens
angDisp: an angular dispersion value
Furthermore, an angular dispersion value in the expression (1) is given by the following expressions:
angDisp1=m/(dxc2x7cos xcex2) xe2x80x83xe2x80x83(2) 
angDisp2=2m/(dxc2x7cos xcex2) xe2x80x83xe2x80x83(3) 
Where each symbol represents the following value.
angDisp1: an angular dispersion value of the single pass beam
angDisp2: an angular dispersion value of the double pass beam
m: an order of diffraction
d: an interval of groove lines of a diffraction grating
xcex2: an output angle from a diffraction grating
Although JP-A-11-132848 discloses no concrete example of diffraction grating 103, an Echelle grating is generally used as a suitable diffraction grating for obtaining such a large diffraction angle as shown in FIG. 10. The Echelle grating is designed so that the diffraction efficiency becomes higher when an incident angle and an output angle are almost the same as a prescribed blaze angle. The blaze angle needs to be prescribed large in order to obtain a large angular dispersion value.
In the present technology level, however, an Echelle grating having a blaze angle of about 80xc2x0 is a marginal one which has been made, and it is very difficult to make an Echelle grating having the blaze angle larger than about 80xc2x0.
Accordingly, as seen from the expressions (1) to (3), when an Echelle grating is used as diffraction grating 103, it can not be expected to improve resolving power of the spectrometer as shown in FIG. 10 by changing an angular dispersion value of the single pass beam or the double pass beam.
On the other hand, as seen from the expression (1), it is also possible to improve resolving power of the spectrometer as shown in FIG. 10 by lengthening a focal length of collimator lens 101. However, in this case, at least the space between slit board 100 and collimator lens 101 needs to be widened, and the system scale would be enlarged while resolving power can be improved.
The present invention has been accomplished in view of these problems. The first object of the present invention is to provide a spectrometer that can obtain fine information (information in the vicinity of a peak) with high accuracy and also obtain coarse information (information in a foot part around the peak) from a spectrum having specified wavelength components that are included in a light beam supplied from a light source without requiring an economical burden or troublesome labor. Moreover, the second object of the present invention is to provide a spectrometer which can realize high resolving power without enlarging a size of the apparatus.
In order to solve the above-mentioned problems, a spectrometer according to a first aspect of the present invention is used for measuring a spectrum of a light beam supplied from a light source, and comprises collimating means for changing a light beam supplied from a light source into a parallel light beam; first diffraction means for diffracting a specified wavelength component included in the parallel light beam into a predetermined direction; second diffraction means, which can be moved between a first position and a second position at least, for diffracting the parallel light beam output from the first diffraction means toward the first diffraction means so that the parallel light beam goes and returns K times, where K represents a natural number, between the first diffraction means and the second diffraction means; detection means for detecting the parallel light beam output from the first diffraction means, which has gone and returned K times between the first diffraction means and the second diffraction means; and control means for changing a value of K by moving the second diffraction means from one of the first position and the second position to the other at least.
In the above-mentioned constitution, for example, the first position is determined so that the parallel light beam which has gone and returned one time between the first diffraction means and the second diffraction means is focused on the detection means. On the other hand, the second position is determined so that the parallel light beam which has gone and returned two times between the first diffraction means and the second diffraction means is focused on the detection means. The parallel light beam which has gone and returned one time between the first diffraction means and the second diffraction means is used for obtaining coarse information of the spectrum. On the other hand, the parallel light beam which has gone and returned two times between the first diffraction means and the second diffraction means is used for obtaining fine information of the spectrum.
Further, a spectrometer according to a second aspect of the present invention is used for measuring a spectrum of a light beam supplied from a light source, and comprises collimating means for changing a light beam supplied from a light source into a parallel light beam; first diffraction means for diffracting a specified wavelength component included in the parallel light beam into a predetermined direction; second diffraction means for diffracting the parallel light beam output from the first diffraction means toward the first diffraction means so that the parallel light beam goes and returns between the first diffraction means and the second diffraction means; first detection means for detecting the parallel light beam output from the first diffraction means, which has gone and returned L times, where L represents a natural number, between the first diffraction means and the second diffraction means; and second detection means for detecting the parallel light beam output from the first diffraction means, which has gone and returned M times, where M represents a natural number larger than L, between the first diffraction means and the second diffraction means.
In the above-mentioned constitution, the parallel light beam which has gone and returned L times between the first diffraction means and the second diffraction means is used for obtaining coarse information of the spectrum. On the other hand, the parallel light beam which has gone and returned M times between the first diffraction means and the second diffraction means is used for obtaining fine information of the spectrum.
According to the present invention, fine information from a spectrum having specified wavelength components that are included in a light beam supplied from a light source can be obtained with high accuracy, and coarse information from the same spectrum can be obtained without requiring an economical burden or troublesome labor.
Furthermore, in the conventional spectrometer, a parallel light beam which contains a specified wavelength component of a light beam supplied from a light source is focused on the detection means after diffracted from one diffraction means. On the other hand, according to the present invention, a parallel light beam which contains a specified wavelength component of a light beam supplied from a light source is focused on the detection means after diffracted from both of the first diffraction means and the second diffraction means.
On this account, according to the present invention, the number of times of diffracting a parallel light beam which contains a specified wavelength component included in a light beam supplied from a light source can be made larger than that in the conventional spectrometer. As a result, an angular dispersion value larger than that in the conventional spectrometer can be obtained. That is, a smaller dispersion value can be obtained without lengthening a focal length of the collimating means. Accordingly, higher resolving power can be realized without enlarging a size of the apparatus.